1. Field of the Invention
The invention concerns an arrangement for cooling an electric cable which comprises concentrically arranged inner and outer conductors at high-voltage and zero potential, respectively; is cooled by a coolant to a low temperature; is subdivided into at least two cable section; and is provided at its ends with a cable termination each, in which said inner and outer conductors are connected with the respective inner and outer ordinary conductors.
2. Description of the Prior Art
In order to make the transmission of large amounts of electric power by means of a cable with conductors cooled to a low temperature, particularly by means of superconductors, as economical as possible, high transmission voltages must be provided. They are particularly necessary for a.c. transmission, because the transmission currents are limited by the fact that the magnetic induction at the surface of these conductors should not exceed a given flux density, the so-called "critical" flux density, in order to keep the a.c. losses of these conductors relatively low. To achieve a further increase of the transmitted power, with a conductor arrangement of concentrically arranged, superconducting inner and outer conductors the surface areas and therefore, also the conductor diameter, must then also be increased proportionally with the current. On the other hand, it is advisable to keep the conductor diameter as small as possible, because the thermal losses of the deep-cooled conductor arrangement increase with the surface area because of radiation and thermal inflow by way of the customary mounting and support devices. Also the relatively high cost of the superconductive material of the conductors and the desire to keep the right-of-way for the cable as narrow as possible lead to small conductor diameters.
The cooling losses of the cable terminations for such cables are determined essentially by the operating current of the cable, because the latter determines in the terminations the cross sections of the ordinary conductors to be optimized on the basis of the Joule losses and thermal inflow from about 300.degree. to 4.degree. K. For this reason it is advisable to choose the transmission voltage for a given transmission power as high as possible, so that the current can be kept as low as possible.
The maximum operating voltage of a cable with a coaxial conductor arrangement, particularly an a.c. cable, and of the cable terminations connected with it is determined not only by the attainable dielectric strength of the conductor insulation and its dielectric losses, but also by the requirements of the cooling loops necessary therefor. In a cooling loop for cooling the superconductors of this cable, it must be taken into consideration that the inner conductor is at high-voltage potential. As the coolant advantageously cools the superconductors by direct contact, it also assumes this potential. When feeding in the coolant for the inner conductor, the full voltage must therefore be overcome. Similar conditions supply also to the cable terminations. Their inner ordinary conductors, which are at high-voltage potential, and which are connected with the superconductors as well as also with a high-voltage feed line, are cooled by direct contact with the coolant. This insures good heat transfer and favorable heat removal. These cable terminations therefoe require coolant supply and discharge lines which must be laid out for the full potential difference.
The temperature range for cooling the superconductors of such an a.c. cable is relatively small, because the a.c. losses increase with temperature and must in general be limited. For niobium, for instance, a cooling range between 4.2.degree. and 6.degree. K must be provided, while the cooling range for niobium-tin, where the a.c. losses are higher, can extend from about 4.5.degree. to 10.degree. K. In contrast, the temperature range for cooling the ordinary conductors of the cable terminations is from about 4.2.degree. to about 300.degree. K, corresponding to the temperature gradient of these normal conductors. Such a temperature transition can be accomplished, for instance, by a bath of boiling helium, the evaporating helium of which rises at the ordinary conductors, which may have the form of wires or laminations, and cools them. Such a design is known, for instance, from The Review of Scientific Instruments, vol. 38, no. 12, December 1967, pages 1776 to 1779.
Because of the different temperature ranges and the different state and pressure conditions, one starts out generally with providing separate cooling loops for the cable line as well as for the cable terminations, in order to obtain favorable matching to the refrigeration machines necessary to operate the cable line. The coolant for the cable is generally fed from one end of the cable. At the cable termination of the cable line at the output end, however, one therefore has othehr operating temperatures and operating conditions than at the cable terminations at the input side. As good thermal contact axists, furthermore, with the normal conductors of the terminations by means of the leads to the superconductors of the cable, special measures must be taken that these contact points in the cable terminations have the same temperature as the cooling loop for the superconductors of the cable has at these points. Otherwise, nonuniformity of the cooling of the superconductors or the ordinary conductors of the cable terminations can occur. The cable terminations would then be cooled by means of the cooling loop for the superconductors of the cable, or vice versa, this cooling loop would be cooled by way of the cable terminations. For this purpose, pressure regulating devices can, for instance, be used for controlling the temperature of the coolant baths required for the normal conductors of the cable terminations as well as control devices for influencing the coolant flow rate for the inner and outer conductors of the cable line.
It is therefore an object of the invention to simplify the arrangement for cooling the cable mentioned at the outset with a cable termination at each end as described above, and in particular, to eliminate the difficulties mentioned and to make economical operation of the cable line possible.